The invention concerns a zeolitic catalytic composition and its use, in particular, in a process for selective reduction of nitrogen oxides contained, for example, in combustion fumes.
Nitrogen oxides, and nitric oxide in particular, are pollutant gases which form during combustion of carbonaceous matter such as coal, petroleum products and natural gas, in thermal power plants for example.
Various processes have been suggested to eliminate these noxious products of combustion gases, for example by absorption and by selective reduction to nitrogen and water vapor. The latter method, generally considered to be the most effective, uses ammonia in the presence of a catalyst as a selective reducer.
Numerous catalysts have been suggested for the realization of this reaction. A good summary of the present state of technology is given in the article by J. R. Kiovsky et al., Ind. Eng. Chem. Prod. Res. Dev. 1980, 19, 218-225.
Among these are:
the noble metals such as platinum supported on alumina and silica-alumina, PA1 Cu, Fe, Cr and Ni compounds supported on alumina, PA1 V, W, Mo and Ce oxides supported on alumina, silica-alumina and titanium oxide, PA1 various natural and synthetic zeolites in proton form or exchanged with various cations, notably Cu. PA1 parasitic oxidation of sulfur dioxide to sulfur trioxide, leading to corrosion of plants, PA1 lack of stability with the course of time, notably in the presence of sulfur dioxide, PA1 oxidation of some of the ammonia by the oxygen contained in fumes which leads to overconsumption of ammonia. This is usually accompanied by formation of nitrogen oxides, and nitrous oxide N.sub.2 O in particular, which prevents high elimination rates of nitrogen oxides being obtained. PA1 difficulty in obtaining exhaustive elimination of nitrogen oxides without notable leakage of ammonia into the fumes.
Patent EP 234974 which describes large-pore mordenites and patent EP 141917 which mentions use of a ceramic molecular sieve, thus of an amorphous structure, illustrate the technological background, and U.S. Pat. No. 4,663,300, which contains a mordenite with titanium, and the summary Chemical Abstract, vol 87, No. 2, 11/7/1977, page 325, No. 10890 a, indicate use of a mordenite associated with clay and containing iron, chromium and/or vanadium.
Most of these catalysts present a number of disadvantages which limit their performance when they are used in the presence of combustion gases. These disadvantages are mainly:
To remedy some of these disadvantages, various solutions have been suggested. Thus, the U.S. Pat. No. 4,473,535 shows that use of a mordenite-based catalyst exchanged with Cu allows better conversion of NO to be obtained than with a platinum-based catalyst on alumina, and that the former does not give rise to parasitic oxidation of NH.sub.3 to NO when the temperature is increased. However, this patent does not provide any details concerning the other problems mentioned hereinabove, that is to say, leakage of NH.sub.3, formation of N.sub.2 O, stability in the presence of SO.sub.2 and formation of SO.sub.3. Furthermore, the examples given relate exclusively to use of a Norton HZ 900 mordenite of the large-pore type. This is also the case in the patent FR 2197815.